Doctors have long known that preterm birth increases children’s risk of developing attention deficit hyperactivity disorder (ADHD), but now a new study of more than a million children in Sweden strengthens the association.

Researchers found that babies born about a month early, at 35 and 36 weeks — who account for the majority of preterm births — were 30% more likely to be treated for ADHD upon reaching school age, compared with children born full-term (at 39 to 41 weeks). The risk increased with the degree of prematurity: babies born at 33 to 34 weeks were 40% more likely to have ADHD than full-term babies, and those born at 29 to 32 weeks were 60% more likely. Babies born even earlier than that — between 23 and 28 weeks — had twice the risk.

The study [PDF], which was published in Pediatrics, looked at government records for 1,180,616 babies born between 1987 and 2000. The researchers cross-referenced birth records with records of prescriptions for ADHD medications in 2006 — which, for the purposes of the study, were used to determine whether a child had ADHD.

Notably, in a second analysis, researchers also compared ADHD rates in preterm children with those of their full-term siblings, born to the same mother. That comparison allowed researchers to separate out effects of preterm birth from genetic and environmental influences.

In total, 7,506 children were prescribed medication to treat ADHD between the ages of 6 and 19. The association between timing of birth and risk of ADHD persisted after researchers controlled for other risk factors including maternal age, education, smoking status, single parenthood, public assistance and family history of psychiatric disorders.

However, the researchers found that the effect of moderate preterm birth (at 33 to 36 weeks) on ADHD was heightened in families where the mother had low education. “In this study, low maternal education is most probably a marker for a complex web of risk factors associated with social adversity, such as relative poverty, living in lowstatus housing areas and family discord,” the authors wrote. “Thus, it is not surprising that the risk of ADHD caused by moderately preterm birth was modified by the socioeconomic context of the family in which the child had been brought up.”

According to background information in the study, ADHD is the most common neurodevelopmental disorder in Western countries (3% to 5% of Swedish children have it), but its exact causes are still unknown. Researchers agree that risk factors include genes (though they haven’t been identified) as well as environmental factors, particularly exposure in the womb to toxic substances like lead, alcohol and PCBs (the industrial pollutant polychlorinated biphenyl). Other environmental influences include poverty and childhood maltreatment.

The current study used drug prescriptions as a stand-in for ADHD, which the authors say is a valid indicator of more severe cases in Sweden. But even in that country, access to medications varied between counties — as it certainly does in other countries.

“A more important source of bias in this study is probably the special access to care in follow-up programs for the most vulnerable patients from NICUs. It seems likely that this may lead to a better access to qualified care for ADHD-symptoms for the most preterm, but probably not the moderately preterm, compared with the general population,” the authors wrote.